Reduction of ferrocene

The sulfonic acids of these metallocenes can be converted to sulfonyl chlorides, sulfonamides, etc., by appropriate reagents. Reduction of ferrocene-sulfonyl chloride by lithium aluminum hydride produces the rapidly oxidized ferrocenethiol in quantitative yield (43). Both the sulfonic acid of cyclopentadienylmanganese tricarbonyl and the corresponding sulfinic acid (obtained by sodium sulfite reduction of the sulfonic acid) have been converted to sulfones (10). 

In addition to stereoselective metalation, other methods have been applied for the synthesis of enantiomerically pure planar chiral compounds. Many racemic planar chiral amines and acids can be resolved by both classical and chromatographic techniques (see Sect. 4.3.1.1 for references on resolution procedures). Some enzymes have the remarkable ability to differentiate planar chiral compounds. For example, horse liver alcohol dehydrogenase (HLADH) catalyzes the oxidation of achiral ferrocene-1,2-dimethanol by NAD to (S)-2-hydroxymethyl-ferrocenealdehyde with 86% ee (Fig. 4-2la) and the reduction of ferrocene-1,2-dialdehyde by NADH to (I )-2-hydroxymethyl-ferrocenealdehyde with 94% ee (Fig. 4-2lb) [14]. Fermenting baker s yeast also reduces ferrocene-1,2-dialdehyde to (I )-2-hydroxymethyl-ferro-cenealdehyde [17]. HLADH has been used for a kinetic resolution of 2-methyl-ferrocenemethanol, giving 64% ee in the product, (S)-2-methyl-ferrocenealdehyde... 

Substituted ferrocenes also form monoanions at very negative potentials electron addition is genuinely associated with the ferrocene nucleus rather than with an electroactive substituent. The E° value for [Fe(f/-C5H4Ph)2] is — 2.62 V, and [FeCp2] itself shows a quasi-reversible reduction at —2.93 V in dmf (452), with a peak separation of 250 mV at — 37°C(v = 1 Vsec-1). Exhaustive electrolytic reduction of ferrocene derivatives yields solutions containing the substituted cyclopentadienide anions the latter may be used in the syntheses of other cyclopentadienylmetal complexes (453). Ferrocenes are also finding use as mediators in electron-transfer reactions, especially at electrode surfaces (454-456). 

Diastereospecific reduction of ferrocenic alcohols by ionic hydrogenation. 

Cyclic voltammetric behaviour of redox polymers including PVF has been studied previously in acetonitrile and in water solutions [18]. In acetonitrile, PVF exhibits stable, symmetrically shaped cyclic voltammetry peaks at potentials characteristic of oxidation and re-reduction of ferrocene sites in the polymer film. In aqueous electrolyte solutions, non-symmetrical peaks are evident in both anodic and cathodic branches. Differences in PVF behaviour in the two solvents have been attributed to solvent uptake in the polymer film (lower for aqueous solutions), changes in site-site interaction parameters for the polymer film (attractive for aqueous electrolytes and repulsive for acetonitrile electrolytes), and differences in deswelling processes in aqueous solution in the reduction half of the cycle as compared with the oxidation half (Figure 2.4). Acetonitrile is a better swelling solvent for PVF than water [18] and break in of the spin-coated films usually requires more cycling in water than in acetonitrile. 

In organic electrolyte (Figure 3a), a well-defined and stable redox process was observed and assigned to the oxidation and reduction of ferrocene groups immobilized on the surface of MOF particles. The position of the oxidation and reduction peaks was independent of the scan rate, indicative of fast electron transfer. However, the value of ca. 100 mV for A p indicates (Table 1) that at least one of the species involved in the redox reaction is not attached to the surface. The peak current for the oxidation of ferrocene in [Zn40(bdc-NH2)(btb)4/3] was indeed found to be proportional to which is a result in-between the theoretical predictions for diffusion-limited (v ) and surface-confined reaction (v). For ferrocene in Al(OH)(bdc-NH2), the exponent value of 0.56 was found, in agreement with the theoretical value of 1/2 expected for a diffusion-limited process. 

In view of the problem of polyalkylation which arises in these direct alkylation reactions, monoalkylated ferrocenes are best prepared by indirect routes. Suitable methods include the reduction of ferrocene esters and acyls with lithium aluminium hydride [316, 317, 318], e.g. 

Fukuzumi S, Okamoto K, Gros CP, Guilard R. 2004. Mechanism of four-electron reduction of dioxygen to water by ferrocene derivatives in the presence of perchloric acid in benzonitrile, catalyzed by cofacial dicobalt porphyrins. J Am Chem Soc 126 10441. 

These observations for Fe(Cp)2 in the presence of the polymer-bound Cr complex are consistent with Fe(Cp)2+, generated electrochemically, undergoing a reaction with Cr(CN-[P])c resulting in the chemical reduction of Fe(Cp)2+ and oxidation of the Cr species. Therefore, when the cathodic part of the Fe(Cp)2 /Fe(Cp)2+ wave is scanned, little ferricenium ion remains to be reduced electrochemically. As a result, the ferrocene molecule has effected the transfer of electrons from the polymer to the electrode. 

Compound 6 contains seven iron-based units [ 12], of which the six peripheral ones are chemically and topologically equivalent, whereas that constituting the core (Fe(Cp)(C6Me6)+) has a different chemical nature. Accordingly, two redox processes are observed, i.e., oxidation of the peripheral ferrocene moieties and reduction of the core, whose cyclic voltammetric waves have current intensities in the 6 1 ratio. Clearly, the one-electron process of the core is a convenient internal standard to calibrate the number of electron exchanged in the multi-electron process. In the absence of an internal standard, the number of exchanged electrons has to be obtained by coulometry measurements, or by comparison with the intensity of the wave of an external standard after correction for the different diffusion coefficients [15]. 

Fu has reported a planar-chiral bisphosphorus ligand 45 with a phosphaferrocene backbone. The ligand has provided enantioselectivity up to 96% ee in the hydrogenation of a-dehydroamino acid derivatives.99 Another planar-chiral ferrocene-based bisphosphorus ligand 46 has been reported by Kagan recently and enantioselectivity up to 95% ee has been obtained in the reduction of dimethyl itaconate.100... 

Fig. 4. The effect of added electrolyte (Bu4N+ salts) on the reduction of approximately 20mM [Co(dmg)3(BF)2]BF4 by ferrocene at —20°C in CH2C12. The solid lines represent the parameters fit to all of the data, as described in the text. From top to bottom Br-, Cl-, BF, N03. In all cases, the rate constant in the absence of ion pairing, from the fitting, is 2.8 xlO5 M-1s-1. Reproduced from Ref. (5) by permission of Elsevier Science.

Kumada s use of a ferrocene moved away from the C2-symmetrical motive, as planar chirality can result from the two ferrocene rings having different substituents. The development of this class of ligand is well documented [5, 125-127]. The best-known uses of these ligands are for reductions of carbon-heteroatom multiple bonds, as in the synthesis of the herbicide, Metolachlor [128, 129]. 

With ferrocenes, an alternative approach has been to attach the phosphorus moieties only to side chains. The WalPhos family (36) forms an eight-mem-bered chelate with the metal. Members of this family provide good selectivity and reactivity for the reductions of dehydroamino and itaconic acid derivatives as well as a,/ -unsaturated carboxylic acids [145, 156],... 

The rhodium complexes of the ferrocene derivatives 39 have shown useful characteristics for the reduction of itaconates as well as dehydroamino acid derivatives [15, 167-170]. These compounds are hybrids between ferrocene-based ligands and the various other types. The P-chiral compounds, which in some ways are DIPAMP hybrids, showed tolerance for the reduction of N-methyl en-amides to produce N-methyl-a-amino acid derivatives [169-171]. 

DuPhos. The exception for rhodium-catalyzed reductions are CnrPhos and BPE-4 [168, 264—268]. MalPhos has proven useful for the reductions of yS-acylamino-acrylates [260]. The ferrocene hybrid (FerroTANE) was referred to earlier (see Section 23.4.1) [167, 222]. The PennPhos ligand is useful for the reductions of cyclic enamides and enol acetates both classes of compounds are difficult for DuPhos itself to reduce with high selectivity [269, 270]. 

Ferrocene-based complexes have some potential for the enantioselective reduction of ketones, but compared to other ligand classes this is relatively limited [3]. Rh complexes of bppfa, bophoz and josiphos are among the most selective catalysts for the hydrogenation of a-functionalized ketones (Table 25.9 Fig. 25.18, 30-32). Ru complexes of walphos and ferrotane are quite effective for... 

Fig. 18b.9. Example cychc voltammograms due to (a) multi-electron transfer redox reaction two-step reduction of methyl viologen MV2++e = MV++e = MV. (b) ferrocene confined as covalently attached surface-modified electroactive species—peaks show no diffusion tail, (c) follow-up chemical reaction A and C are electroactive, C is produced from B through irreversible chemical conversion of B, and (d) electrocatalysis of hydrogen peroxide decomposition by phosphomolybdic acid adsorbed on a graphite electrode.

In agreement with such an electronic distribution, the cyclic voltammogram of ferrocene displays an oxidation profile (peak A) which is accompanied in the reverse scan by a directly associated reduction process (peak B), Figure 4. 

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